The cellulose industry utilizes many different processes and apparatus, such as for thickening, in which the pulp is discharged at a high consistency of from about 8 to about 25%. It is normal practice to direct the pulp either to a mass tower, to a drop leg or a suction chamber, the latter two being of smaller size, and wherefrom the pulp is transferred by pumping for further processing. Displacement type high consistency pumps are traditionally used for this kind of pumping.
Recently, the tendency has been to replace displacement pumps by specially constructed centrifugal pumps, which provide several advantages compared to displacement pumps, e.g. centrifugal pumps are of smaller size, have greater flexibility of capacity, and have little need for service.
Usually, the above mentioned types of pumps also operate rather well with pulp having considerably high consistencies, provided the flow of pulp through the suction chamber or drop leg is even and the pulp is homogeneous in quality. In practice, this is, however, not the case when the pulp flows to the suction side of the pump, for example, from disk or drum thickeners or washers. Generally, the pulp received from such apparatus contains fairly large plank-like lumps. This kind of non-homogeneous and lumpy pulp will easily adhere to and/or clog the drop leg by forming arch-like formations in front of the suction opening of the pump thus preventing the pulp, having already a rather low consistency, from flowing into the pump inlet. Normally, pumping at such consistencies and with the above-mentioned pumps is thus rendered very difficult and often impossible. Additionally, this kind of pulp includes great amounts of air which is harmful to the pumping operation and the further processing thereof.
Efforts have been made to solve the above-mentioned problems by using different types of screw conveyors which are arranged to feed the pulp from the bottom of the suction chamber to the suction opening of the pump. Also, with this kind of feed apparatus movement of the non-homogenous pulp is often impeded above the feed apparatus so that the pulp at high consistency, accumulates in arch-like formations. Attempts have been made to solve this problem with displacement pumps by sufficiently overdimensioning the pump so that the screw conveyor operating as a feed apparatus of the pump is practically empty during the operation thereof, whereby the pulp is intended to drop directly onto the screw conveyor and thus prevent arching or immobilization of the pulp in the drop leg. This effort results, in addition to the large size and high price of the pump, in the entry of air into the pulp to be pumped; the air being of course harmful to the process. Additionally, there is always the risk of the drop leg becoming only partially filled and the pulp getting stuck, if, for example, the pulp flowing from the drum thickener loosens unevenly and drops in large planks.
Furthermore, a screw is known which is used in mass towers and drop legs and which is provided with a thread parallel to the shaft of the tower, mostly the vertical shaft, and which is open from the inside and by which the pulp pillar or column is transferred downwardly. With such arrangement, if used together with a bottom wiper in the tower, it is at best possible to prevent the arching of the pulp in front of the suction opening of the pump, but this method frequently results in lumping of the pulp. Additionally, this large screw requires additional power which stands in no relation to the benefits achieved.